1. Field of the Invention
This invention relates to a lens barrel having a zooming function.
2. Description of the Related Art
The so-called motor-operated zoom lens barrel and the motor-operated zoom camera of the kind arranged to shift a focusing lens by a motor power and to shift a power varying lens (or a variator lens) also by a motor power have been commercialized. However, the use of the focusing lens and the variator lens not only has increased the sizes of the known motor-operated zoom lens barrel and that of the motor-operated zoom camera but also has increased their prices.
Meanwhile, a motor-operated zoom lens barrel and a motor-operated zoom camera which are arranged to have the focusing lens and the variator lens to be driven by a common motor have been proposed. Examples of such arrangement have been disclosed in U.S. Pat. No. 3,370,907 and Japanese Laid-Open Patent Application No. SHO 60-263911. The example disclosed in this U.S. Patent is arranged as described below:
There are provided a switching operation part which is arranged to make change-over between a focusing action and a zooming action and a switch which is arranged as follows: In the case of the focusing action, the switch causes the focusing lens to be driven and moved toward the nearest distance position thereof when the motor is rotated forward and toward an infinite distance position when the motor is rotated backward. In the case of the zooming action, the switch causes the variator lens to be driven and moved toward a telephoto end position when the motor is rotated forward and toward a wide-angle end position when the motor is rotated backward. The switching operation part and the switch are discretely arranged. Therefore, the proposed arrangement has necessitated a complicated operation. Further, in this case, nothing is disclosed in respect of automatically performing the focusing action. However, even if a focusing mechanism is arranged to automatically carry out the focusing action, the troublesome zooming operation would still remain unimproved because the switch which selects the telephoto direction or the wide-angle direction in zooming must be separately arranged from the focusing/zooming selecting operation part.
Meanwhile, the example disclosed in Japanese Laid-Open Patent Application No. SHO 60-263911 is arranged as follows: Switching between a focusing action and a zooming action is arranged to be effected through a control circuit in accordance with an instruction given from a selection switch which is provided for selection between the zooming toward a telephoto-end position and the zooming toward a wide-angle end position. That arrangement, however, necessitates the use of a solenoid, a clutch mechanism which acts in association with the solenoid and a solenoid control circuit besides the switch for selection of either the telephoto direction or the wide-angle direction. This results in an increase in cost. In addition to this, the solenoid must be energized every time the zoom switch is operated to select the telephoto direction or the wide-angle direction. Therefore, electric energy consumption increases. As a result, a battery must be replaced often.
Further, a lens barrel which has a lens holding frame arranged to be axially shiftable by rotating it is known. In the case of this known lens barrel, the lens holding frame is driven to rotate by a driving gear which is secured to a shaft disposed within the lens barrel. The lens holding frame is provided with a driven gear which is arranged in one body with the frame to engage the driving gear. The above-stated shaft is arrangedwithin the lens barrel not to be movable in the axial direction. To ensure that the driving gear and the driven gear slidingly move relative to each other in the axial direction while they are retained in an engaged state in driving the lens holding frame, the axial length (or the tooth width) of the driven gear is arranged to be longer than the length of the stroke of the relative movement of these gears, i.e., the length of the stroke of the lens holding frame. However, the conventional lens barrel which is arranged in this manner has presented the following problems:
(i) As mentioned above, the axial length (or tooth width) of the driven gear must be arranged to be long, including at least the stroke length of the lens holding frame. This causes the lens holding frame to be longer than a length actually necessary and thus results in an increase in the inertial mass of the frame. In addition to that, the long frame causes an increase in the size of the whole lens barrel.
(ii) The necessity of arranging the driving and driven gears to make the relative axial sliding movement in an engaged state necessitates their teeth to be designed to have a large engaging clearance between them. This prevents these gears from being engaged with each other with a high degree of precision and prevents efficient power transmission. Further, the lens holding frame sometimes unevenly rotates. The lens, therefore, cannot be moved and positioned at a high degree of precision. This problem is serious, particularly in cases where the lens barrel has a telephoto lens system of a large rate of magnification.